1. Technical Field
The present invention relates to a printed wiring board manufacturing method, and in particular to a printed wiring board manufacturing method whereby a circuit pattern is formed on the outer surface of an insulating material.
2. Background of the Art
<<Technological Background>>
A printed wiring board, for example, a flexible printed wiring board has made remarkable progress in precision, thickness, and weight. In particular, high density and miniaturization of a formed circuit pattern is outstanding.
The printed wiring board is formed with a large number of minute through holes for electrically conducting a circuit pattern on both sides and for mounting semiconductor parts.
Two major methods, namely panel plating and button plating for such through-hole fabrication and electrical conduction, are used during the process for manufacturing the printed wiring board.
<<Panel Plating Method>>
Referring to a panel plating method, a copper foil is attached to both sides (i.e., front side and rear side) of an insulating material to form a substrate. After a number of through holes is made in the substrate, the insides of the through holes are made electrically conductive before copper electroplating the entire substrate.
According to the panel plating method, a circuit pattern is formed on the substrate, of which the inside of the through holes is made electrically conductive by copper-plating, by going through known patterning steps of exposing, developing, etching, and separating in that order. In this manner, a printed wiring board is manufactured.
However, according to such a panel plating method, not only the inside of the through holes, but also the copper foils on both sides are copper-plated, and as a result, the circuit patterns on both sides of the manufactured printed wiring board are also copper-plated. Thus, a drawback whereby not only flexibility and bendability deteriorate, but also weight increases has been pointed out and this has been regarded as a notable problem in the flexible printed wiring board.
<<New and Old Button Plating Methods>>
A button plating method has been developed as a method for overcoming such a drawback and is taking root in a field where more emphasis is put on lightness in weight as well as flexibility and bendability.
This button plating method comprises the steps of providing a number of through holes in a substrate; making the inside of the through holes electrically conductive; coating the substrate with a photosensitive dry film; applying a photomask, which is a negative mask, on the outside of the photosensitive dry film, and exposing and developing the photosensitive dry film as a plating resist; and copper-electroplating the inside of the opened through holes and the opening periphery of the through holes as a button section in a substantially button shape.
According to such a button plating method, the substrate of which the through holes and the like have been copper-plated is formed with a circuit pattern by following the patterning steps and thus, a printed wiring board is formed.
However, in this conventional button plating method (hereinafter referred to as “Old Button Plating Method”), it has been pointed out that the location of each through hole of the substrate and the location of each corresponding part of the photomask (and a specific electroplating location) are often out of position. In other words, according to this conventional button plating method, such an incorrect positioning or displacement is often caused by the expansion and contraction of the substrate and the photomask and the difficulty in a visual positioning operation between the substrate and the photomask and this displacement has resulted in a disconnection defect and the like.
In view of this problem, an inventor has improved the conventional button plating method and filed a patent application with a Japanese Patent Office under the serial number of 2004-291202 (hereinafter simply referred to as “New Button Plating Method”).
According to this new button plating method, a developing solution in place of the photomask is poured into the through holes from the other side of the photosensitive dry film which is coated on one side of the substrate, the photosensitive dry film is then developed as a plating resist and is hardened by exposure, and copper-electroplating is conducted. As a result of adopting this new button plating method, the displacement problem pointed out in the old button plating method can be eliminated and occurrence of the disconnection defect resulting from the displacement can also be avoided.
<<Document Information on the Conventional Technology>>
An example of the old button plating method is disclosed in the following Patent Document 1.
Patent Document 1: Japanese Unexamined Patent Publication No. Hei 11-195849